The goal is to gain further understanding of the spinothalamic tract, one of the principle pathways conveying information about painful stimuli to the brain. The cells of origin and thalamic terminations of the dorsal spinothalamic tract (DSTT), which travels in the dorsolateral quadrant of the spinal cord, will be determined in monkeys and cats, and compared to the origins and terminations of the classical ventral spinothalamic tract (VSTT), which travels in the ventrolateral quadrant of the spinal cord. Retrograde and anterograde transport of horseradish peroxidase and flourescent tracers will be used to anatomically map the locations of the cells of origin and terminations of these two separate pathways. In addition, recordings will be made from the lateral sensory thalamus in anesthetized cats and monkeys, and the relative contributions of the VSTT and the DSTT to the responses of these units elicited by cutaneous stimulation will be determined by transiently blocking transmission through either the dorsolateral or the ventrolateral quadrant of the spinal cord. The DSTT has only recently been discovered and is likely one of the major nociceptive specific inputs to the thalamus. Consequently gaining a better understanding of this system will allow new insights into the processing of painful stimuli. The adequate treatment of both acute and chronic pain states in humans clearly depends on indepth knowledge about the pathways that carry information concerning peripheral painful stimuli from the spinal cord to the thalamus.